Aircraft manufacturers and fuel suppliers continue to seek improved emission quality and improved fuel economy. Many aircraft engines are able to meet current emission standards using combustor technologies and theories developed over the past 50 years of engine development. However, stricter engine emission standards may not be within the capability of current combustor technologies.
Air pollution concerns worldwide have led to stricter emission standards both domestically and internationally. Aircraft emissions are governed by both Environmental Protection Agency (EPA) and International Civil Aviation Organization (ICAO) standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (UHC), and carbon monoxide (CO) from aircraft engines especially in the vicinity of airports, where they contribute to urban photochemical smog problems.
In general, engine emissions fall into two classes: emissions formed as a result of high flame temperatures, such as NOx, and emissions formed as a result of low flame temperatures that do not allow the fuel-air reaction to proceed to completion, such as UHC and CO. Since different emissions are formed at different temperatures, it is difficult to mitigate simultaneously NOx, UHC, and CO.
For the fuel economy, fuels costs are a major concern for United States airline industry. In 2007, United States passenger and cargo airline operations required 19.6 billion gallons of jet fuel, or approximately 465 million barrels. The significantly increasing cost of fuel is causing the airline industry to reexam the business models upon which they operate.